1. Field of the Invention
The invention relates to the field of propulsion systems for aircraft and, in particular, to a shroud for a reversible thrust fan.
2. Description of Related Art
Fan shrouds are traditionally designed to have low external drag and provide high pressure and high quality flow to the forward thrusting fan. Additionally, the shroud is generally designed to provide these high efficiency characteristics at cruise conditions, where the aircraft spends most of its flight time. For conventional subsonic transport aircraft that cruise at high altitude and speed, a moderately thin and low radius entry lip is acceptable for both cruise and for takeoff and low speed flight. For a conventional and STOVL supersonic aircraft, a thin sharp entry lip of minimum radius is required for high speed operation. Such an entry lip is adequate to supply a sufficient quantity of air to the engine during high speed flight because of the ramming effect due to the velocity of the aircraft. However, such an entry lip generally doesn't have sufficient radius to supply a sufficient quantity of air to the engine during take-off and low-speed flight. For a STOVL subsonic aircraft, the entry lip used for conventional subsonic transport aircraft is acceptable for cruise but a large radius "fat" entry lip of a bellmouth type is preferred to prevent lip loss turbulence and loss of boundary layer control during takeoff, hover, and vertical landing.
Thus auxiliary air intakes are often incorporated into the shroud inlet. These can take the form of simple passively operated "blow" in doors that are spring biased to the closed position. Such doors open in response to a decrease in the internal pressure within the inlet duct that occurs when the fan draws in air during take-off. As speed increases and ram air effects take hold, the pressure within the shroud increases causing the doors to close. Often more sophisticated systems are incorporated, such as the disclosed in U.S. Pat. No. 3,664,612, "Aircraft Engine Variable Highlight Inlet" by W. E. Skidmore, et al. In this design two airfoils are pivotally mounted at the leading edge of the shroud that are extended to form an aerodynamically clean auxiliary intake. Another approach is to incorporate an extendible lip to form an auxiliary inlet passage. Such a system is disclosed in U.S. Pat. No. 5,014,933, "Translating Lip Aircraft Cowling Structure Adapted For Noise Reduction" by D. J. Harm, et al. In both instances, the main thrust of the inventions is to improve the low speed efficiency of a fan shroud inlet designed for high speed operation. The effectiveness of such devices are discussed in articles such as AIAA-81-1396 "Zero Length Inlets For Subsonic V/STOL Aircraft" by (AIAA/SAE/ASME 17th Joint Conference, Jul. 27-29, 1989, and AIAA-80-1245 "Zero Length Slotted-Lip Inlet For Subsonic Military Aircraft" (AIAA/SAE/ASME 16th Joint Conference, Jun. 30-Jul. 2, 1980 both by E. R. Glasgow, et al.
For fans operating in the low speed range, such as found on lighter-than-air vehicles that operate in the 70 knot range, not only is it important to have an inlet entry lip with a large radius to reduce flow separation, it is equally important to have a gentle external boattail slope to reduce pressure drag and a sharp trailing edge to eliminate nozzle base drag. With the overall speed range so low, no efficiency enhancing auxiliary devices to increase flow into the inlet of the shroud are necessary. However, if reverse thrust operation of the fans is going to be used to aid in critical airship docking maneuvers, wherein the exhaust nozzle becomes the inlet, severe losses in fan efficiency will be experienced. In fact, in some instance fan efficiency is reduced by 80 percent. However, the inlet to the shroud performs fairly well as an exhaust nozzle. Consideration has been given to rotating the nacelles 180 degrees to eliminate this unacceptable loss in propulsive efficiency. However, the reduced response time in critical docking maneuvers is, in many instances, unacceptable.
Thus it is a primary object of the subject invention to provide a system for increasing the efficiency of a fan shroud designed for use with a fan normally operating in a low forward speed range when providing reverse thrust.
It is another primary object of the subject invention to provide a passive system for increasing the efficiency of the fan shroud designed for use with a fan operating in a low forward speed range when providing reverse thrust.
It is a further object of the subject invention to provide a passive system for increasing the efficiency of a fan shroud designed for use with a fan operating in a low forward speed range when providing reverse thrust that does not significantly interfere with shroud performance when the fan is operating in the normal forward thrust mode.